Now a hitherto known hot-water storage type hot-water supply apparatus operating under the natural circulation principle will be explained with reference to FIG. 1 of the attached drawings. As shown in the drawing, water contained within a hot-water storage tank (3) provided with a water feed orifice (1) and a hot-water supply orifice (2) is fed through a first circulation pipe (4) connected to the bottom of tank (3) to a heat exchanger (5) to be heated by a burner or a combustor (6), the water thus heated being supplied through a second circulation pipe (7) to the upper portion within tank (3).
In this case, assuming that the mean density of water flowing through first circulation pipe (4) is r.sub.1, the mean density of hot-water flowing through second circulation pipe (7) is r.sub.2, and the height from the middle position of heat exchanger (5) to the boundary surface between the hot-water and the cold-water within hot-water storage tank (3) is H.sub.1, the natural circulation force F is formulated as follows: EQU F=(r.sub.1 -r.sub.2).multidot.H.sub.1 ( 1)
It has also been known that the flow rate w of the cold- or hot-water passing through a circulation passage A comprising first circulation pipe (4), heat exchanger (5) and second circulation pipe (7) is related to the natural circulation force F by the following equation: EQU w.varies.F.sup.n (n.gtoreq.1) (2)
Further, assuming that the heat quantity given to the water in heat exchanger (5) by the heat from combustor (6) is Q, the specific heat at constant pressure is C.sub.p, and each of the temperatures of the cold- and hot-water at the entrance and the exit of heat exchanger (5) are Tin and Tout, respectively, the next equation is given: EQU w=Q/C.sub.p (Tout-Tin) (3)
Therefore, the equations (2) and (3) become as follows: EQU 1/(Tout-Tin).varies.F.sup.n ( 4)
Thus, it will be appreciated that the smaller the natural circulation force F, the larger the temperature difference (Tout-Tin) becomes.
In short, as shown in FIG. 1, if the hot-water accumulates as high within hot-water storage tank (3) as a level denoted by the dot-and-dash line B.sub.1, height H decreases to as low as H.sub.1, while (r.sub.1 -r.sub.2) increases a bit, but, since the degree of decrease in H is larger than the degree of increase in (r.sub.1 -r.sub.2), the natural circulation force F as represented by formula (1) decreases so that the temperature difference (Tout-Tin) in the temperature of the water before and after the heat exchanger becomes large. This means that the larger the hot-water volume accumulated in the upper part of hot-water tank (3), the larger the temperature difference (Tout-Tin) becomes. From the above reasons, the temperature distribution of the hot-water in hot-water storage tank (3) of the conventional hot-water supply apparatus at the time of the boiling of the hot-water can be expressed by the diagram shown in FIG. 2, revealing a temperature gradient wherein the low temperature water exists at the bottom of the tank in the initial stage of the hot-water storage, and the higher the level of the hot-water from the bottom of tank 3, the higher the temperature of the hot-water becomes. Therefore, it has such defect that the volume of the high temperature hot-water available for practical use, i.e. the effective hot-water storage volume is less than the actual inner volume of the tank.
And, at the time of the supply of the hot-water, since, with the supply of hot-water, the circulation force F, i.e. the discharge volume from second circulation pipe (7), varies, the boundary surface between the hot-water and the cold-water varies so that the actuation and the shutting off of combustor (6) are repeated. In this case, since at the time of the actuation of the combustor the volume of the hot-water accumulated in the hot-water storage tank is small, the natural circulation force F becomes large, and the temperature of the hot-water flowing out of second circulation pipe (7) becomes low, so the temperature of the hot-water to be supplied also becomes low. Inversely, since at the time of the shutting off of the combustor the volume of the hot-water accumulated in the hot-water storage tank is large, the natural circulation force F is small, so the high temperature hot-water is supplied. Thus, as shown in FIG. 3 the variation in temperature of the hot-water at the time of its supply is large. Therefore, the conventional hot-water supply apparatus is not suited for use with, for example, a shower bath, etc.